In the pumping of liquid, such as waste water comprising solid matter, by means of, for instance, a submersible pump, the solid matter will sooner or later adversely affect the capacity of the pump to transport liquid. The solid matter is caught in the hydraulic unit of the pump and adheres slowly to the impeller of the pump as well as to the inside of the pump housing of the pump, and thereby the hydraulic efficiency of the pump is adversely affected and the pump will operate in a strained operating condition as a consequence of increased rotation resistance, increased moment of inertia, and impaired hydraulic properties. At present, there are several known methods for more or less automatic cleaning of a pump when the pump, or more precisely the hydraulic unit of the pump, begins clogging. The strained operating condition is not detrimental to the pump, but a higher current consumption and inferior pump performance are obtained, which is expensive for the plant owner and which may involve adverse consequences such as flooded pump station when the available capacity of the pump is not enough for emptying the pump station.
Known cleaning methods, or methods for controlling a pump arrangement, are relatively rough and lack capacity to analyse the load factor of the motor and what consequences different load factors may have. Known cleaning methods detect that cleaning is required and then carry out a predetermined standard cleaning sequence, which at least involves that the motor of the pump is braked by the fact that the rotational speed of the motor is subjected to an extended, predetermined down-ramping driven by the control unit. It is known that it is not wanted/recommended to stop the motor of the pump abruptly, above all because of requirements to avoid so-called water hammer in the pipe system downstream the pump, but also because of the large moment of inertia and the large momentum possessed by the impeller of the pump in normal operation. If the motor is stopped abruptly, water hammer arises inevitably where the kinetic energy of the liquid and moment of inertia in the pipe conduits downstream the pump create vibrations that risk destroying the pipe conduits and other engineering components, and moreover, the risk is imminent that the impeller comes loose, the drive shaft of the pump is damaged, etc. Thus, an extended, controlled down-ramping of the rotational speed of the motor always takes place.
A direct consequence of the lack of intelligence of the cleaning method is that the standard cleaning sequence used, and which is adequate in strained operating conditions as described above, drastically increases the load factor of the pump when a large and/or hard object enters the hydraulic unit of the pump and is wedged up, i.e., when an operating condition detrimental to the pump arrangement has arisen. With detrimental operating condition, reference is made to an operating condition that immediately or in the short term will cause the pump and/or the control unit to break. When the control unit, for instance in the form of a frequency converter (VFD), carries out said down-ramping when a large and/or hard object has wedged and mechanically brakes the impeller, the extended, controlled down-ramping causes the motor to force the impeller to rotate and the object is wedged harder/more severe. This causes in turn the impeller, drive shaft, motor, etc., of the pump or the control unit to become overworked and damaged.
In order to prevent the pump and/or the control unit from being damaged, various security systems/protective equipment are used today, such as protective motor switches, fuses, etc., which are arranged to protect the equipment and be triggered before the equipment is damaged. Common to the detrimental operating conditions described above, i.e., if security systems are triggered and/or if the pump arrangement breaks, it is required that service staff makes an emergency turn-out and attends to the error/clogging. These turnouts are expensive per se and, moreover, an inoperative pump is expensive for the plant owner.